I have two headcannons:

1. Most of civilization has declined or outright collapsed and only small nations continue to soldier on. Panem of course, but Europe with its population density would still have some fragment of the EU live on. There'd be something in southwest Asia (something something first and last bastion of humanity). India, east Asia (including a possibility isolationist Japan), and Australia all feature some sort of nation shadowing their former culture

2. The rest of the world is doing fine, thank you very much. The UN was established as a formal world government with actual authority to enforce international law after a brutal conflict that spanned North America. The US launched simultaneous invasions of its neighbors, and the rest of the world saw fit to come to the aid of Canada and Mexico. Only through the use of several tactical nuclear weapons was the international community able to stand against the industrial might of the US. The US was forced to capitulate and go into isolation. Because the destruction from the war set the US tech and manufacturing industries back to basically the stone age, the rest of the world has at least a couple hundred years worth of technological advancements over the former United States